1. Field of the Invention
The present invention relates to a method and product of the same, wherein the method comprises converting Si to SiO.sub.2 in a directional fashion whereby steam or wet oxidation of Si is enhanced by the prior implantation of Ge into the Si. During the oxidation, the Ge is piled-up ahead of the SiO.sub.2 /Si interface. This segregation of Ge leads to the formation of a distinct, almost pure Ge layer which is epitaxial with the underlying Si. The U.S. Government has rights in this invention pursuant to Contract No. DE-AB01-87GC20109.M000 awarded by the U.S. Department of Energy.
2. Description of Related Art
The fabrication of integrated circuits in Si (silicon) depends critically on the ability of Si to form a thermal oxide. Not only are such oxides used as implantation and diffusion masks, but also to electrically isolate devices and various device components. Thermal oxides also function as the gate dielectric in MOSFET's (metal-oxide-semiconductor field-effect transistors). The importance of thermal oxides in the entire integrated circuit (IC) fabrication process warrants investigation of the basic oxidation mechanism and means to control the process for reproducible growth of thin, high-quality oxides. Thermal oxidation in Si occurs by the diffusion of oxidant (oxidizing specie) through the oxide to the oxide/Si interface where it reacts with Si to form SiO.sub.2. Gotzlich et al, "Dopant Dependence . . . ," Radiation Effects, 47, pp. 203-210 (1980). However, this is accomplished slowly and is nondirectional in nature, causing a condition known as "bird beaking" in IC manufacturing. This lateral growth of SiO.sub.2 could be limited by enhancing the vertical growth. The enhancement of oxide growth could also reduce the amount of heat and time required for the oxidation step. A selective Ge.sup.+ implantation of Si (accomplished by standard lithography techniques) could produce varying thicknesses of oxidation on the same Si wafer. This could be beneficial in forming, using a single oxidation cycle, oxides of different thickness for device and masking applications. A specific masking application which utlizes the differential oxide growth rates is detailed later. It is known that the presence of dopants in the near-surface region of Si can effect oxidation rates. Previous work, i.e. Gotzlich et al, has shown that Group III and V dopants can enhance the rate of oxidation of Si. However, prior-art impurities are electrically active and can have deleterious effects on IC performance when present in the field oxide or in the underlying Si. An electrically-inactive impurity is therefore desirable.
There is evidence that a rough interface between Si and SiO.sub.2 can be deleterious to the electrical qualities of the SiO.sub.2. If the interface could be made smoother and/or flatter, the quality of the SiO.sub.2 properties would be increased. The rough interface appears to be related to strain caused by the volumetric mismatch between the Si and the oxide phase which forms. Accommodation of this mismatch is a suspected cause for the injection of point defects (e.g. interstitials) into the Si substrate during oxidation which leads to extended defects and stacking faults that getter metallic impurities in the Si, creating deleterious pockets of conductivity, or "leaks." If such defects could be reduced, IC reliability would be substantially increased. Thus, a method for enhancing oxide growth on Si is desirable. Also, elmination of point-detect injection during oxidation will eliminate the enhanced diffusion in the underlying Si substrate that many dopants experience as a result of the non-equilibrium defect population.
It is also known that if Ge (germanium) could be epitaxially formed on a Si substrate, several new and useful configurations and methods relating to electronic circuits could result. Ge and Si devices could be built into the same integrated circuit (IC). Devices using both Ge and Si could be manufactured. An epitaxial layer of Ge on Si would have advantages in forming epitaxial GaAs on Si substrates for use in optical coupling circuitry.
Germanium has other advantages for use in semiconductors, one of which is its higher mobility as compared with Si. However, a native oxide layer is not practical with Ge because GeO.sub.2 is water soluble and is not structurally stable enough to provide the protection that is purposed by its presence. A stable oxide layer is needed to make Ge semiconductor applications more practical.
Epitaxial Ge can be produced by techniques such as molecular beam epitaxy, Appl. Phys. Lett., 49(5), pp. 286-88 (1986). However, such techniques require elaborate cleaning procedures and ultrahigh vacuum systems.